Semiconductor device

ABSTRACT

A semiconductor device includes a first-type internal stress film formed of a silicon oxide film over source/drain regions of an nMISFET and a second-type internal stress film formed of a TEOS film over source/drain regions of a pMISFET. In a channel region of the nMISFET, a tensile stress is generated in the direction of movement of electrons due to the first-type internal stress film, so that the mobility of electrons is increased. In a channel region of the pMISFET, a compressive stress is generated in the direction of movement of holes due to the second-type internal stress film, so that the mobility of holes is increased.

BACKGROUND OF THE INVENTION

The present invention relates to a semiconductor device including anMISFET and a method for fabricating the same, and more particularlyrelates to a measure for increasing the mobility of carriers.

When a stress is generated in a semiconductor crystal layer, acrystal-lattice constant varies and a band structure is changed, so thatthe mobility of carriers is changed. This phenomenon has been known asthe “piezo resistivity effect”. Whether the carrier mobility isincreased or reduced differs depending on the plane direction of asubstrate, the direction in which carriers move, and whether the stressis a tensile stress or a compressive stress. For example, in an Si (100)substrate, i.e., a silicon substrate of which the principal surface isthe {100} plane, assume that carriers move in the [011] direction. Whencarriers are electrons, with a tensile stress generated in the directionin which electrons in a channel region move, the mobility of thecarriers is increased. On the other hand, when carriers are holes, witha compressive stress generated in the direction in which holes in achannel region move, the mobility of the carriers is increased. Theincrease rate of carrier mobility is proportional to the size of astress.

In this connection, conventionally, there have been proposals forincreasing carrier mobility by applying a stress to a semiconductorcrystal layer to increase the operation speed of transistors and thelike. For example, in Reference 1, an entire semiconductor substrate isbent using an external device, thereby generating a stress in an activeregion of a transistor.

SUMMARY OF THE INVENTION

However, in the above-described known structure, an external device isneeded in addition to a semiconductor substrate and a stress can begenerated only in the same direction in an entire region of thesemiconductor substrate in which active regions of a transistor and thelike are provided and which is located in the principal surface side.For example, when an Si (100) substrate is used, neither the mobility ofelectrons nor the mobility of holes can be increased.

It is therefore an object of the present invention to provide, bygenerating a stress which increases the mobility of carriers in asemiconductor layer without using an external device, a semiconductordevice including a pMISFET and an nMISFET of which respective operationspeeds are increased and a method for fabricating the same.

A semiconductor device according to the present invention includes aninternal stress film for generating a stress in a gate length directionin a channel region of an active region in which a MISFET is formed.

Thus, the mobility of carriers in the MISFET can be increased by usingthe piezo resistivity effect.

The internal stress film is capable of covering one or both ofsource/drain regions. In an nMISFET, the internal stress film generatesa tensile stress substantially in the parallel direction to a gatelength direction in a channel region (i.e., the direction of movement ofelectrons). In a pMISFET, the internal stress film generates acompressive stress substantially in the parallel direction to a gatelength direction in a channel region (i.e., the direction of movement ofholes).

Covering both side surfaces or both side and upper surfaces of a gateelectrode, the internal stress film can generate a stress in thelongitudinal direction of the channel region through the gate electrode,thereby increasing the mobility of carriers.

Moreover, covering a side surface of the gate electrode and an uppersurface of the semiconductor substrate in two regions of the substratesandwiching part of the gate electrode, whether the MISFET is an nMISFETor a pMISFET, the internal stress film can generate a tensile stresssubstantially in the parallel direction to the gate width direction ofthe MISFET, thereby increasing the mobility of carriers.

A first method for fabricating a semiconductor device according to thepresent invention is a method in which an nMISFET and a pMISFET areformed in first and second active regions of a semiconductor substrate,respectively, and then first and second internal stress films whichcover source/drain regions of the nMISFET and source/drain regions ofthe pMISFET, respectively, and generate a tensile stress and acompressive stress, respectively, substantially in the paralleldirections to respective gate length directions of the channel regionsare formed.

According to this method, a CMOS device of which the operation speed isincreased can be obtained.

A second method for fabricating a semiconductor device according to thepresent invention is a method in which an internal stress film is formedfirst, a groove is formed in the internal stress film, a gate insulatingfilm and a buried gate electrode are formed in the groove, and then theinternal stress film is removed.

According to this method, a stress which increases the mobility ofcarriers in the channel region can be generated using a remaining stressin the gate insulating film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a semiconductor deviceaccording to a first embodiment of the present invention.

FIG. 2A through 2C are cross-sectional views illustrating first half ofrespective steps for fabricating the semiconductor device of the firstembodiment.

FIG. 3A through 3C are cross-sectional views illustrating latter half ofrespective steps for fabricating the semiconductor device of the firstembodiment.

FIGS. 4A through 4C are cross-sectional views illustrating first, secondand third modified examples of the first embodiment.

FIGS. 5A through 5D are cross-sectional views illustrating respectivesteps for fabricating a semiconductor device according to the firstmodified example of the first embodiment.

FIGS. 6A through 6C are cross-sectional views illustrating respectivesteps for fabricating a semiconductor device according to the thirdmodified example of the first embodiment.

FIGS. 7A through 7D are cross-sectional views illustrating first half ofrespective steps for fabricating a semiconductor device according to asecond embodiment of the present invention.

FIGS. 8A through 8D are cross-sectional views illustrating latter halfof respective steps for fabricating the semiconductor device of thesecond embodiment.

FIGS. 9A and 9B are a plane view of an MISFET of a semiconductor deviceaccording to a third embodiment of the present invention and across-sectional view illustrating a cross-sectional structure takenalong the line IX-IX (a cross section in the gate width direction),respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

FIG. 1 is a cross-sectional view illustrating a semiconductor deviceaccording to a first embodiment of the present invention. As shown inFIG. 1, a surface region of a semiconductor substrate 1, i.e., an Si(100) substrate is divided into a plurality of active regions 1 a and 1b by an isolation region 2. The semiconductor device includes an nMISFETformation region Rn which includes the active region 1 a and in which annMISFET is to be formed and a pMISFET formation region Rp which includesthe active region 1 b and in which a pMISFET is to be formed.

The nMISFET includes n-type source/drain regions 3 a and 4 a each ofwhich includes an n-type lightly doped impurity region, an n-typeheavily doped impurity region and a silicide layer such as a CoSi₂layer, a gate insulating film 5 formed on the active region 1 a and madeof a silicon oxide film, a silicon oxynitride film or the like, a gateelectrode 6 a formed on the gate insulating film 5 and made ofpolysilicon, aluminum or the like, and a sidewall 7 covering a sidesurface of the gate electrode 6 a and made of an insulating film. Partof the active region 1 a located under the gate electrode 6 a is achannel region 1 x in which electrons move (travel) when the nMISFET isin an operation state.

The pMISFET includes p-type source/drain regions 3 b and 4 b each ofwhich includes a p-type lightly doped impurity region, a p-type heavilydoped impurity region and a silicide layer such as a CoSi₂ layer, a gateinsulating film 5 formed on the active region 1 b and made of a siliconoxide film, a silicon oxynitride film or the like, a gate electrode 6 bformed on the gate insulating film 5 and made of polysilicon, aluminumor the like, and a sidewall 7 covering a side surface of the gateelectrode 6 b and made of an insulating film. Part of the active region1 b located under the gate electrode 6 b is a channel region 1 y inwhich holes move (travel) when the pMISFET is in an operation state.

Moreover, provided are a first-type internal stress film 8 a formed onthe source/drain regions 3 a and 4 a of the nMISFET, made of a siliconnitride film or the like, and having a thickness of about 20 nm, asecond-type internal stress film 8 b formed on the source/drain regions3 b and 4 b of the pMISFET, made of a TEOS film or the like, and havinga thickness of about 20 nm, an interlevel insulating film 9 covering thenMISFET and pMISFET and having a surface flattened, a lead electrode 10formed on the interlevel insulating film 9, and a contact 11 connectingeach of the source/drain regions 3 a, 3 b, 4 a and 4 b with the leadelectrode 10 through the interlevel insulating film 9.

Herein, an “internal stress film” is a film characterized in that wherethe internal stress film is directly in contact with some other memberor faces some other member with a thin film interposed therebetween, astress is generated in the film itself. As for stress, there are tensilestress and compressive stress. In this embodiment and other embodiments,an internal stress film in which a tensile stress is generatedsubstantially in the parallel direction to the direction in whichcarriers move (i.e., the gate length direction) in a channel region ofan MISFET is referred to as a “first-type internal stress film” and aninternal stress film in which a compressive stress is generatedsubstantially in the parallel direction to the direction in whichcarriers move (the gate length direction) in a channel region of anMISFET is referred to as a “second-type internal stress film”.

Herein, the semiconductor substrate 1 is an Si substrate of which theprincipal surface is the {100} plane and is referred to as an Si (100)substrate for convenience. However, the {100} plane is a general namefor the (±100) plane, the (0±10) plane and the (00±1) plane, andtherefore, even a plane which is not exactly the {100} plane and istilted from the {100} plane by a less angle than 10 degree is consideredto be substantially the {100} plane. Moreover, in this embodiment, thedirection in which electrons move in the nMISFET and the direction inwhich holes move in the pMISFET (i.e., the gate length direction of eachMISFET) is the [011] direction. However, in this embodiment, the “[011]direction on the principal surface of an Si (100) substrate” includesequivalent directions to the [011] direction, such as the [01-1]direction, the [0-11] direction, and the [0-1-1] direction, i.e.,directions within the range of the <011> direction. That is, even adirection which is not exactly the [011] direction and tilted from the<011> direction by a less angle than 10 degree is considered to besubstantially the [011] direction.

According to this embodiment, the following effects can be obtained.

In the nMISFET, when the first-type internal stress film 8 a is broughtinto a direct contact with a semiconductor layer or made to face asemiconductor layer with a thin film interposed therebetween, a stressfor compressing the first-type internal stress film itself, i.e., acompressive stress is generated in the first-type internal stress film 8a. As a result, by the first-type internal stress film 8 a, thesemiconductor layer adjacent to the first-type internal stress film 8 acan be stretched in the vertical direction to a boundary surface.Specifically, the first-type internal stress film 8 a applies acompressive stress to the source region 3 a and the drain region 4 a inthe active region 1 a of the nMISFET in the parallel direction to theprincipal surface. As a result, a tensile stress is applied to a regionof the substrate located between the source region 3 a and the drainregion 4 a, i.e., the channel region 1 x in the gate length direction(the direction in which electrons move when the nMISFET is in anoperation state). Then, with this tensile stress, electrons areinfluenced by the piezo resistivity effect, so that the mobility ofelectrons is increased. Herein, “substantially in the paralleldirection” also means in a direction tilted by an angle of less than 10degree from the direction in which electrons move.

For example, assume that the substrate 1 is an Si (100) substrate andthe direction in which electrons move is the [011] direction. When theinternal stress of the first-type internal stress film 8 a adjacent tothe semiconductor layer is a general level for a silicon nitride film,i.e., 1.5 GPa, the thickness of the first-type internal stress film 8 ais 20 nm, a space between respective parts of the source and drainregions 3 a and 4 a being in contact with the first-type internal stressfilm 8 a, i.e., the length of the channel region 1 x, is 0.2 μm, atensile stress in the gate length direction generated at a depth of 10nm from the surface of the substrate is 0.3 GPa (J. Appl. Phys., vol.38-7, p. 2913, 1967) and the improvement rate of the mobility ofelectrons is +10% (Phys. Rev., vol. 94, p. 42, 1954). To obtain a largerchange in the mobility than this, the tensile stress of a semiconductorcan be increased. Thus, a film having a large internal stress can beused as the first-type internal stress film 8 a, the thickness of thefirst-type internal stress film 8 a can be increased, or the spacebetween the parts of the source and drain regions 3 a and 4 a being incontact with the first-type internal stress film 8 a, i.e., the lengthof the channel region 1 x, can be reduced for a larger change in themobility. For example, when the thickness of the first-type internalstress film 8 a is doubled, the space between the parts of the sourceand drain regions 3 a and 4 a being in contact with the first-typeinternal stress film 8 a, i.e., the length of the channel region 1 x isreduced to half, the improvement rate of the mobility of electrons is+40%. As another way to obtain a large mobility, the direction in whichelectrons move is changed from the [011] direction to the [010]direction to change the improvement rate of the mobility of electronswith respect to a tensile stress. As a result, with the same tensilestress, the improvement rate of the mobility becomes about 3.5 timeslarge. Although the source and drain regions 3 a and 4 a receivecompressive stresses by the first-type internal stress film 8 a,influence of the piezo resistivity effect is small because alow-resistant heavily doped semiconductor device and a silicide film areused. Moreover, influence of the internal stress of the interlevelinsulating film 9 on the channel region can be neglected. This isbecause with the substrate covered by the interlevel insulating film 9,internal stresses in the interlevel insulating film 9 are cancelled offwith each other, so that the function of applying stress to the activeregions 1 a and 1 b is small.

In the pMISFET, when the second-type internal stress film 8 b is broughtinto a direct contact with the semiconductor layer or made to face asemiconductor layer with a thin film interposed therebetween, a stressfor stretching the second-type internal stress film itself, i.e., atensile stress is generated in the second-type internal stress film 8 b.As a result, by the second-type internal stress film 8 b, thesemiconductor layer adjacent to the second-type internal stress film 8 bis compressed in the vertical direction to a boundary surface.Specifically, the second-type internal stress film 8 b applies a tensilestress to the source region 3 b and the drain region 4 b in the activeregion 1 b of the pMISFET in the parallel direction to the principalsurface. As a result, a compressive stress is applied to a region of thesubstrate located between the source region 3 b and the drain region 4b, i.e., the channel region 1 y substantially in the parallel directionto the gate length direction (the direction in which holes move when thepMISFET is in an operation state). Then, with this compressive stress,holes are influenced by the piezo resistivity effect, so that themobility of holes is increased. Herein, “substantially in the paralleldirection” also means in a direction tilted by an angle of less than 10degree from the direction in which electrons move.

Note that, instead of the internal stress films 8 a and 8 b, thesemiconductor film itself in which the source and drain regions 3 a, 4a, 3 b and 4 b are formed may be a film having an internal stress, forexample, an uppermost semiconductor layer in an SOI substrate.

Furthermore, each of the internal stress films 8 a and 8 b does not haveto be a single layer but may include multiple layers, as long as each ofthe internal stress films 8 a and 8 b can apply a stress to thesubstrate as a whole.

Moreover, in this embodiment, an Si (100) substrate is used. However,even if an Si (111) substrate is used, with the direction in whichelectrons move set to be the [001] direction, the mobility of electronsis increased by a tensile stress. In general, in any substrate planedirections, there is a direction of movement of electrons or holes,which allows increase in the mobility of electrons or holes according tothe direction of a stress.

In this embodiment, the internal stress films 8 a and 8 b exist on thesource/drain regions 3 a and 4 a and the source/drain regions 3 b and 4b, respectively. However, even when the internal stress film 8 a existsonly on one of the source/drain regions 3 a and 4 a and the internalstress film 8 b exists only on one of the source/drain regions 3 b and 4b, the effect of increasing the mobility of carriers can be obtained. Inthis case, the improvement rate of the mobility is reduced to half. Ineach of the following embodiments, when an internal stress film exitsonly on one of source/drain regions, the improvement rate of themobility is reduced to half, compared to the case where internal stressfilms exist on source/drain regions, but the mobility is increased.

FIGS. 2A through 2C and FIGS. 3A through 3C are cross-sectional viewsillustrating respective steps for fabricating a semiconductor deviceaccording to the first embodiment of the present invention.

First, in the process step of FIG. 2A, a trench and a buried oxide filmare formed in part of a semiconductor substrate 1, i.e., an Si (100)substrate, thereby forming an isolation region 2 for dividing thesubstrate into active regions 1 a, 1 b and so on. Thereafter, after agate insulating film 5 has been formed by thermal oxidation ofrespective surfaces of the active regions 1 a and 1 b and a polysiliconfilm for forming gate electrodes has been deposited, the polysiliconfilm and the gate insulating film 5 are etched by patterning usinglithography and anisotropic dry etching, thereby forming gate electrodes6 a and 6 b. The gate length direction of each of the gate electrodes 6a and 6 b is the [011] direction. Next, using the gate electrode 6 a ofthe nMISFET as a mask, ion implantation of an n-type impurity (e.g.,arsenic) at a low concentration is performed to an nMISFET formationregion Rn at an injection energy of 10 keV and a dose of 1×10¹³/cm², andusing the gate electrode 6 b of the pMISFET as a mask, ion implantationof a p-type impurity (e.g., boron) at a low concentration is performedto a pMISFET formation region Rp at an injection energy of 2 keV and adose of 1×10¹⁵/cm². Thereafter, an insulating film which is for forminga sidewall and has a thickness of about 50 nm is deposited on thesubstrate and then a sidewall 7 is formed on side surfaces of the gateelectrodes 6 a and 6 b by etch back. Next, using the gate electrode 6 aof the nMISFET and the sidewall 7 as masks, ion implantation of ann-type impurity (e.g., arsenic) at a high concentration is performed tothe nMISFET formation region Rn at an injection energy of 20 keV and adose of 1×10¹⁴/cm², and ion implantation of a p-type impurity (e.g.,boron) at a high concentration is performed to the pMISFET formationregion Rp at an injection energy of 5 keV and a dose of 1×10¹⁶/cm².Thereafter, thermal treatment (RTA) for activating impurities isperformed. By the above-described processing, source/drain regions 3 aand 4 a including an n-type lightly doped impurity region and an n-typeheavily doped impurity region are formed in the nMISFET formation regionRn and source/drain regions 3 b and 4 b including a p-type lightly dopedimpurity region and a p-type heavily doped impurity region are formed inthe pMISFET formation region Rp.

Next, in the process step of FIG. 2B, a silicon nitride film 8 x isformed on the substrate so that the silicon nitride film 8 x has arelatively large thickness and a surface thereof is flatted. At thispoint of time, the silicon nitride film 8 x covers respective uppersurfaces of the gate electrodes 6 a and 6 b of the MISFETs. Thereafter,a resist film 12 is formed on the silicon nitride film 8 x bylithography and the silicon nitride film 8 x is patterned using theresist film 12 as a mask so that the silicon nitride film 8 x is leftonly on the nMISFET formation region Rn.

Next, in the process step of FIG. 2C, after the resist film 12 has beenremoved, the silicon nitride film 8 x is etched back, part of thesilicon nitride film 8 x located on the gate electrode 6 a is removedand the thickness of the silicon nitride film 8 x is further reduced.Thus, a first-type internal stress film 8 a is formed. That is, thefirst-type internal stress film 8 a does not exist on the gate electrode6 a of the nMISFET but exits only on the source/drain regions 3 a and 4a.

Next, in the process step of FIG. 3A, a TEOS film 8 y is formed on thesubstrate so that the TEOS film 8 y has a relatively large thickness anda surface thereof is flatted. At this point of time, the TEOS film 8 ycovers respective upper surfaces of the gate electrodes 6 a and 6 b ofthe MISFETs. Thereafter, a resist film (not shown) is formed on the TEOSfilm 8 y by lithography and the TEOS film 8 y is patterned using theresist film as a mask so that the TEOS film 8 y is left only on thepMISFET formation region Rp.

Next, in the process step of FIG. 3B, after the resist film has beenremoved, the TEOS film 8 y is etched back, parts of the TEOS film 8 ylocated on the gate electrodes 6 a and 6 b are removed and the thicknessof the TEOS film 8 y is further reduced. Thus, a second-type internalstress film 8 b having substantially the same thickness as that of thefirst-type internal stress film 8 a is formed. That is, the second-typeinternal stress film 8 b does not exist on the gate electrode 6 b of thepMISFET and the first-type internal stress film 8 a but exists only onthe source/drain regions 3 b and 4 b.

By the above-described process steps, the internal stress films 8 a and8 b for applying stresses in opposite directions to each other areformed on the source/drain regions 3 a and 4 a of the nMISFET and thesource/drain regions 3 b and 4 b of the pMISFET, respectively.

Next, in the process step of FIG. 3C, on the substrate, an interlevelinsulating film 9 is formed and then contact holes are formed so as topass through the interlevel insulating film 9 and reach the source/drainregions 3 a and 4 a of the nMISFET by lithography and dry etching, thesource/drain regions 3 b and 4 b, and the gate electrodes 6 a and 6 b,respectively. Thereafter, each of the contact holes is filled with metal(e.g., tungsten), thereby forming contact plugs 11. Furthermore, a metalfilm such as an aluminum alloy film is deposited on the interlevelinsulating film 9 and then the metal film is patterned, thereby forminga lead electrode 10 connected to each of the contact plugs 11. Thus, therespective source/drain regions 3 a, 4 a, 3 b and 4 b of the MISFETs andthe gate electrodes 6 a and 6 b are made to be electrically connectablefrom the outside.

In the fabrication method of this embodiment, either one of the twotypes of internal stress films 8 a and 8 b may be formed first. And theinternal stress films 8 a and 8 b may overlap with each other over theisolation region 2 and the source/drain regions 3 a, 4 a, 3 b and 4 b.

First Modified Example of First Embodiment

FIGS. 4A through 4C are cross-sectional views illustrating first throughthird modified examples of the first embodiment.

A semiconductor device according to a first modified example shown inFIG. 4A has a structure in which the sidewall 7 of the first embodimentis omitted. Moreover, each of the source/drain regions 3 a, 4 a, 3 b and4 b does not include a lightly doped impurity region and includes only aheavily doped impurity region. Other part has the same structure as thatof the semiconductor device of the first embodiment. In this modifiedexample, no sidewall exists in forming an internal stress film, so thata space between respective parts of the source/drain regions 3 a and 4 abeing in contact with the first-type internal stress film 8 a is small.Thus, a stress applied to each of the channel regions 1 x and 1 y isincreased, so that the effect of improving the carrier mobility becomeslarger than that of the first embodiment.

A semiconductor device according to a second modified example shown inFIG. 4B has a structure in which instead of the sidewall 7 of the firstembodiment, which is made of a silicon oxide film, the first-typeinternal stress film 8 a made of a silicon nitride film covers a sidesurface of the gate electrode 6 a of the nMISFET and the second-typeinternal stress film 8 b made of a TEOS film covers a side surface ofthe gate electrode 6 b of the pMISFET. Moreover, each of thesource/drain regions 3 a, 4 a, 3 b and 4 b does not include a lightlydoped impurity region and includes only a heavily doped impurity region.Other part has the same structure as that of the semiconductor device ofthe first embodiment.

In this modified example, in addition to the effect of the firstmodified example, the following effect can be obtained. In the nMISFET,the first-type internal stress film 8 a and the gate electrode 6 a arein contact with each other substantially at the entire side surface ofthe gate electrode 6 a, so that the gate electrode 6 a is compresseddownwardly by the first-type internal stress film 8 a. With the gateelectrode 6 a compressed downwardly, then, in the channel region 1 x, acompressive stress is generated in the vertical direction to theprincipal surface and the mobility of electrodes in the nMISFET isfurther improved.

Moreover, in the pMISFET, the second-type internal stress film 8 b andthe gate electrode 6 b are in contact with each other substantially atthe entire side surface of the gate electrode 6 b, so that the gateelectrode 6 b is stretched upwardly by the second-type internal stressfilm 8 b. With the gate electrode 6 b stretched upwardly, then, in thechannel region 1 y, a tensile stress is generated in the verticaldirection to the principal surface and the mobility of holes in thepMISFET is further improved.

Therefore, in this structure, the improvement rate of the mobility isfurther increased, compared to the first modified example.

A semiconductor device according to a third modified example shown inFIG. 4C has a structure in which instead of the sidewall 7 of the firstembodiment, which is made of a silicon oxide film, the first-typeinternal stress film 8 a made of a silicon nitride film covers side andupper surfaces of the gate electrode 6 a of the nMISFET and thesecond-type internal stress film 8 b made of a TEOS film covers side andupper surfaces of the gate electrode 6 b of the pMISFET. Moreover, eachof the source/drain regions 3 a, 4 a, 3 b and 4 b does not include alightly doped impurity region and includes only a heavily doped impurityregion. Other part has the same structure as that of the semiconductordevice of the first embodiment.

In this modified example, in addition to the effect of the secondmodified example, the following effect can be obtained. In the nMISFET,the first-type internal stress film 8 a and the gate electrode 6 a arein contact with each other substantially at the entire side and uppersurfaces of the gate electrode 6 a, so that the effect of compressingthe gate electrode 6 a downwardly by the first-type internal stress film8 a becomes larger. With the gate electrode 6 a compressed downwardlymore strongly, then, in the channel region 1 x, a compressive stress isgenerated in the vertical direction to the principal surface and themobility of electrons in the nMISFET is further improved.

Moreover, in the pMISFET, the second-type internal stress film 8 b andthe gate electrode 6 b are in contact with each other substantially atthe entire side and upper surfaces, so that the stress of stretching thegate electrode 6 b upwardly by the second-type internal stress film 8 bbecomes larger. With the gate electrode 6 b stretched upwardly morestrongly, then, in the channel region 1 y, a tensile stress is generatedin the vertical direction to the principal surface and the mobility ofholes in the pMISFET is further more improved.

Therefore, in this structure, the improvement rate of the mobility isfurther increased, compared to the second modified example.

In each of the first through third modified examples, only the heavilydoped impurity regions are provided as the source/drain regions.However, the source/drain regions may be source/drain regions includinga lightly doped impurity region and a heavily doped impurity region. Inthis case, as shown in FIG. 2A, a lightly doped impurity region and aheavily doped impurity region are formed using a gate electrode and asidewall, and then, after the sidewall has been removed, an internalstress film is formed in the manner shown in FIGS. 4A through 4C. Thus,source/drain regions including a lightly doped impurity region and aheavily doped impurity region can be formed.

Fabrication Method According to First Modified Example

FIGS. 5A through 5D are cross-sectional views illustrating respectivesteps for fabricating a semiconductor device according to the firstmodified example of the first embodiment.

First, before the process step of FIG. 5A, the same process step as thatof FIG. 2A in the first embodiment is performed to form an isolationregion 2 for dividing a substrate into active regions 1 a, 1 b and soon, a gate insulating film 5, gate electrodes 6 a and 6 b, source/drainregions 3 a and 4 a each including only an n-type heavily doped impurityregion and source/drain regions 3 b and 4 b each including only a p-typeheavily doped impurity region. In this case, each of the source/drainregions 3 a, 4 a, 3 b and 4 b includes only a heavily doped impurityregion. However, the source/drain regions 3 a, 4 a, 3 b and 4 b may besource/drain regions each including a lightly doped impurity region anda heavily doped impurity region. In that case, by the process step ofFIG. 2A, after a heavily doped impurity region has been formed using thesidewall as a mask, the sidewall is removed. Thus, source/drain regionseach including a lightly doped impurity region and a heavily dopedimpurity region can be formed.

Next, in the process step of FIG. 5A, a silicon nitride film 8 x isformed on the substrate so that the silicon nitride film 8 x has arelatively large thickness and a surface thereof is flatted. At thispoint of time, the silicon nitride film 8 x covers respective uppersurfaces of the gate electrodes 6 a and 6 b of the MISFETs. Thereafter,a resist film 12 is formed on the silicon nitride film 8 x bylithography and the silicon nitride film 8 x is patterned using theresist film 12 as a mask so that the silicon nitride film 8 x is left onthe nMISFET formation region Rn.

Next, in the process step of FIG. 5B, after the resist film 12 has beenremoved, the silicon nitride film 8 x is etched back, part of thesilicon nitride film 8 x located on the gate electrode 6 a is removedand the thickness of the silicon nitride film 8 x is further reduced.Thus, a first-type internal stress film 8 a having a thickness of about20 nm is formed. That is, the first-type internal stress film 8 a doesnot exist on the gate electrode 6 a of the nMISFET but exists only onthe source/drain regions 3 a and 4 a.

Next, in the process step of FIG. 5C, a TEOS film 8 y is formed on thesubstrate so that the TEOS film 8 y has a relatively large thickness anda surface thereof is flatted. At this point of time, the TEOS film 8 ycovers respective upper surfaces of the gate electrodes 6 a and 6 b ofthe MISFETs.

Next, in the process step of FIG. 5D, the TEOS film 8 y is etched back,parts of the TEOS film 8 y located on the first-type internal stressfilm 8 a and the gate electrode 6 b of the pMISFET are removed and thethickness of the TEOS film 8 y is further reduced. Thus, a second-typeinternal stress film 8 b having substantially the same thickness as thatof the first-type internal stress film 8 a is formed.

By the above-described process steps, the internal stress films 8 a and8 b for applying stresses in opposite directions to each other areformed on the source/drain regions 3 a and 4 a of the nMISFET and on thesource/drain regions 3 b and 4 b of the pMISFET, respectively.

Although illustration of the subsequent process step will be omitted,the same process step as that of FIG. 3C is performed to form aninterlevel insulating film, contact holes reaching the source/drainregions 3 a and 4 a of the nMISFET, the source/drain regions 3 b and 4 bof the pMISFET, and the gate electrodes 6 a and 6 b, respectively,through the insulating film 9, and contact plugs 11. Furthermore, leadelectrodes are formed on the interlevel insulating film, so that thesource/drain regions 3 a, 4 a, 3 b and 4 b and the gate electrodes 6 aand 6 b of the MISFETs becomes electrically connectable from theoutside.

According to this embodiment, by forming the first-type internal stressfilm 8 a and the second-type internal stress film 8 b, the structureshown in FIG. 4A can be obtained. Thus, no sidewall exists, so that aspace between respective parts of the source/drain regions 3 a and 4 awhich are in contact with the first-type internal stress film 8 a issmall. Accordingly, a stress applied to each of the channel regions 1 xand 1 y is increased, so that the effect of improving the carriermobility becomes larger than that of the first embodiment. The distancebetween the active region 1 a and the first-type internal stress film 8a is reduced, so that a tensile stress generated in the active region isincreased.

Fabrication Method According to Third Modified Example

FIGS. 6A through 6C are cross-sectional views illustrating respectivesteps for fabricating a semiconductor device of the third modifiedexample of the first embodiment.

First, before the process step of FIG. 6A, the same process step as thatof FIG. 2A in the first embodiment is performed to form an isolationregion 2 for dividing a substrate into active regions 1 a, 1 b and soon, a gate insulating film 5, gate electrodes 6 a and 6 b, source/drainregions 3 a and 4 a each including only an n-type heavily doped impurityregion and source/drain regions 3 b and 4 b each including only a p-typeheavily doped impurity region. In this case, each of the source/drainregions 3 a, 4 a, 3 b and 4 b includes only a heavily doped impurityregion. However, the source/drain regions 3 a, 4 a, 3 b and 4 b may besource/drain regions each including a lightly doped impurity region anda heavily doped impurity region. In that case, by the process step ofFIG. 2A, after a heavily doped impurity region has been formed using thesidewall as a mask, the sidewall is removed.

Next, in the process step of FIG. 6A, a silicon nitride film having asmaller thickness than those of the gate electrodes 6 a and 6 b, i.e., athickness of about 20 nm is formed on the substrate. At this point oftime, the silicon nitride film covers side and upper surfaces of thegate electrodes 6 a and 6 b of the MISFETs. Thereafter, a resist film 12is formed on the silicon nitride film by lithography and the siliconnitride film is patterned using the resist film 12 as a mask, therebyforming a compressive stress film 8 a made of the silicon nitride filmonly in an nMISFET formation region Rn.

Next, in the process step of FIG. 6B, after the resist film 12 has beenremoved, a TEOS film having a smaller thickness than those of the gateelectrodes 6 a and 6 b, i.e., a thickness of about 20 nm is formed onthe substrate. At this point of time, the TEOS film covers side andupper surfaces of the gate electrodes 6 a and 6 b of the MISFETs as wellas part of the first-type internal stress film 8 a. Next, a resist film13 is formed on the TEOS film by lithography and the TEOS film ispatterned using the resist film 13 as a mask to remove part of the TEOSfilm located on the gate electrode 6 b of a pMISFET, thereby forming asecond-type internal stress film 8 b which overlaps with the first-typeinternal stress film 8 a over the isolation region 2. At this point oftime, ideally, it is more preferable that the first-type internal stressfilm 8 a and the second-type internal stress film 8 b do not overlapwith each other. However, even though the first-type internal stressfilm Sa and the second-type internal stress film 8 b overlap with eachother over the isolation region 2 or the source/drain regions, a stresswhich the second-type internal stress film 8 b applied to the activeregion 1 a is small because there is a large distance therebetween.

By the above-described process steps, the internal stress films 8 a and8 b for applying stresses in opposite directions to each other areformed on the source/drain regions 3 a and 4 a of the nMISFET and thesource/drain regions 3 b and 4 b of the pMISFET, respectively.

Next, in the process step of FIG. 6C, the resist film 13 is removed andthen the same process step as that of FIG. 3C in the first embodiment isperformed to form an interlevel insulating film 9, contact holesreaching the source/drain regions 3 a and 4 a of the nMISFET, thesource/drain regions 3 b and 4 b of the pMISFET, and the gate electrodes6 a and 6 b, respectively, through the insulating film 9, and contactplugs 11. Furthermore, lead electrodes 10 are formed on the interlevelinsulating film 9, so that the respective source/drain regions 3 a, 4 a,3 b and 4 b and the gate electrodes 6 a and 6 b of the MISFETS can beelectrically accessed from the outside.

The semiconductor device of this modified example has a structure inwhich the first-type internal stress film 8 a made of a silicon nitridefilm covers the side and supper surfaces of the gate electrode 6 a ofthe nMISFET and the second-type internal stress film 8 b made of a TEOSfilm covers the side and upper surfaces of the gate electrode 6 b of thepMISFET. Other part has the same structure as that of the semiconductordevice of the first embodiment.

In this modified example, the following effect can be obtained. In thenMISFET, the first-type internal stress film 8 a and the gate electrode6 a are in contact with each other substantially at the entire side andupper surfaces of the gate electrode 6 a, so that the gate electrode 6 ais strongly compressed downwardly by the first-type internal stress film8 a. With the gate electrode 6 a compressed downwardly, then, in thechannel region 1 x, a compressive stress is generated in the verticaldirection to the principal surface. In this embodiment, the Si (100)substrate is used and the direction of movement of electrons is the[011] direction, so that the mobility of electrons in the nMISFET isfurther improved by this compressive stress.

Moreover, in the pMISFET, the second-type internal stress film 8 b andthe gate electrode 6 b are in contact with each other substantially atthe entire side and upper surfaces of the gate electrode 6 b, so thatthe gate electrode 6 b is strongly stretched upwardly by the second-typeinternal stress film 8 b. With the gate electrode 6 b stretchedupwardly, then, in the channel region 1 y, a tensile stress is generatedin the vertical direction to the principal surface. In this embodiment,the Si (100) substrate is used and the direction of movement of holes isthe [011] direction, so that the mobility of holes in the pMISFET isfurther improved by this tensile stress.

Furthermore, in the fabrication method of this embodiment, in theprocess step of forming the first-type internal stress film 8 a of asilicon nitride film and the process step of forming the second-typeinternal stress film 8 b of a TEOS film, patterning is performed,instead of etch back. Therefore, the fabrication method is moresimplified.

Second Embodiment

FIGS. 7A through 7D and FIGS. 8A through 8D are cross-sectional viewsillustrating respective steps for fabricating a semiconductor deviceaccording to a second embodiment of the present invention.

First, in the process step of FIG. 7A, a trench and a buried oxide filmare formed in part of a semiconductor substrate 1, thereby forming anisolation region 2 for dividing the substrate into active regions 1 a, 1b and so on.

Next, in the process step of FIG. 7B, a silicon nitride film is formedon the substrate so as to have a larger thickness than that of a gateelectrode. At this point of time, a gate electrode of each MISFET is notformed and thus the silicon nitride film covers each of the activeregions 1 a and 1 b and the isolation region 2. Thereafter, a resistfilm 12 is formed on the silicon nitride film by lithography and thenthe silicon nitride film is patterned using the resist film 12 as amask, thereby forming a first-type internal stress film 8 a made of thesilicon nitride film only in an nMISFET formation region Rn.

Next, in the process step of FIG. 7C, a TEOS film is formed on thesubstrate so as to have a larger thickness than that of a gateelectrode. At this point of time, a gate electrode of each MISFET is notformed and thus the TEOS film covers part of the first-type internalstress film 8 a, the active region 1 b and the isolation region 2.Thereafter, a resist film 13 is formed on the TEOS film and the TEOSfilm is patterned using the resist film 13 as a mask, thereby forming asecond-type internal stress film 8 b made of the TEOS film in a pMISFETformation region Rp. In this case, the first-type internal stress film 8a and the second-type internal stress film 8 b overlap with each otherover the isolation region 2.

Next, in the process step of FIG. 7D, a resist film 14 is formed on thefirst-type internal stress film 8 a and the second-type internal stressfilm 8 b by lithography so as to have openings each corresponding to agate formation region of each MISFET and then the first-type internalstress film 8 a and the second-type internal stress film 8 b are etchedusing the resist film 14 as a mask, thereby forming grooves 6 x and 6 yeach of which reaches an upper surface of the semiconductor substrate 1.In this case, a tensile stress in the lateral direction is generated ina channel region 1 x of the active region 1 a by the first-type internalstress film 8 a and a compressive stress is generated in a channelregion 1 y of the active region 1 b.

Next, in the process step of FIG. 8A, after the resist film 14 has beenremoved, thermal treatment is performed to regions of the semiconductorsubstrate 1 exposed at the respective bottoms of the grooves 6 x and 6y, thereby forming a gate insulating film 5. Next, a polysilicon film 6is deposited in the grooves 6 x and 6 y and on the first-type internalstress films 8 a and the second-type internal stress film 8 b to athickness with which the upper surface of the polysilicon film 6 becomessubstantially flatted.

Next, in the process step of FIG. 8B, the polysilicon film 6 is etchedback to remove parts of the polysilicon film 6 located over thefirst-type internal stress film 8 a and the second-type internal stressfilm 8 b and to fill each of the grooves 6 x and 6 y with thepolysilicon film, thereby forming gate electrodes 6 a and 6 b.Furthermore, lithography and ion implantation are performed to dope thegate electrode 6 a of the nMISFET formation region Rn with an n-typeimpurity and the gate electrode 6 b of the pMISFET formation region Rpwith a p-type impurity, thereby reducing the resistance of each of thegate electrodes 6 a and 6 b.

Next, in the process step of FIG. 8C, after the internal stress films 8a and 8 b have been removed by selective etching, lithography and ionimplantation using each of the gate electrodes 6 a and 6 b as a mask areperformed so that n-type and p-type impurities are injected into thenMISFET formation region Rn and the pMISFET formation region Rp,respectively. Thus, source/drain regions 3 a and 4 a each including onlyan n-type heavily doped impurity region of the nMISFET and source/drainregions 3 b and 4 b each including only a p-type heavily doped impurityregion of the pMISFET are formed. Stresses which have been applied tothe channel regions 1 x and 1 y by the internal stress films 8 a and 8 bare reduced by removing the internal stress films 8 a and 8 b. However,the gate insulating film 6 is formed with stresses generated in thechannel regions 1 x and 1 y, and therefore, the gate insulating film 5holds stresses in the channel regions 1 x and 1 y, i.e., regions of thesubstrate located directly under the gate insulating film 5. Forexample, assume that an Si (100) substrate is used as the semiconductorsubstrate 1 and a silicon oxide film is used as the gate insulating film5. When the internal stress films 8 a and 8 b are removed, each ofstresses in the channel regions 1 x and 1 y is reduced to one fourthbecause the ratio between the respective Young's moduli of the siliconoxide film and the Si (100) substrate is 1:3. To keep each of a tensilestress and a compressive stress at a large level, an insulation materialof which the Young's modulus is larger than that of the Si (100)substrate can be used as the gate insulating film 5. A silicon nitridefilm is an example of insulation materials with a larger Young's modulusthan that of the Si (100) substrate. In this case, each of thesource/drain regions 3 a, 4 a, 3 b and 4 b includes only a heavily dopedimpurity region. However, the source/drain regions 3 a, 4 a, 3 b and 4 bmay be source/drain regions each including a lightly doped impurityregion and a heavily doped impurity region. In that case, after alightly doped impurity region has been formed by ion implantation at alow concentration using a gate electrode as a mask, a sidewall can beformed on a side surface of the gate electrode and then a heavily dopedimpurity region can be formed by ion implantation at a highconcentration using the sidewall as a mask.

Next, in the process step of FIG. 8D, the same process step as that ofFIG. 3C in the first embodiment is performed to form an interlevelinsulating film 9, contact holes reaching the source/drain regions 3 aand 4 a of the nMISFET, the source/drain regions 3 b and 4 b of thepMISFET, and the gate electrodes 6 a and 6 b, respectively, through theinsulating film 9, and contact plugs 11. Furthermore, lead electrodes 10are formed on the interlevel insulating film 9, so that the respectivesource/drain regions 3 a, 4 a, 3 b and 4 b and the gate electrodes 6 aand 6 b of the MISFETs can be electrically accessed from the outside.

In this embodiment, compared to the first embodiment, the followingadvantage can be obtained. In the first embodiment, there is possibilitythat when stresses are generated in the channel regions 1 x and 1 y,stresses to be generated by the first-type internal stress film 8 a andthe second-type internal stress film 8 b in the channel regions 1 x and1 y might be interrupted by the gate electrodes 6 a and 6 b and thenreduced because the gate electrodes 6 a and 6 b exist. In contrast, inthis embodiment, when the grooves 6 x and 6 y are formed in the processstep of FIG. 7D, the semiconductor substrate 1 is the only member whichis in contact with the internal stress films 8 a and 8 b. Therefore, astress applied to each of the channel regions 1 x and 1 y is increased.

Note that in this embodiment, a polysilicon film which is doped with animpurity is used for the gate electrodes 8 a and 8 b. However, if as afilm for forming the gate electrodes 8 a and 8 b, a film which isconductive without being doped with an impurity (for example, a metalfilm such as a copper film, a tungsten film, a cobalt film and a nickelfilm) is used instead of a polysilicon film, the subsequent ionimplantation is not necessary. Therefore, fabrication process steps canbe simplified.

Moreover, in this embodiment, the internal stress films 8 a and 8 b areremoved for the purpose of performing ion implantation for forming thesource/drain regions 3 a, 4 a, 3 b and 4 b. However, if the thicknessesof the internal stress films 8 a and 8 b are small, impurity ions can beinjected into the semiconductor substrate 1 through the internal stressfilms 8 a and 8 b. Therefore, the internal stress films 8 a and 8 b canbe left without being removed.

Moreover, in the process step of FIG. 8A, exposed part of a surfaceportion of the semiconductor substrate is thermally oxidized to form agate insulating film. However, a gate insulating film can be depositedby CVD or PVD. In that case, an insulating film (e.g., a silicon oxidefilm) for forming a gate insulating film is left on a side surface ofeach of the gate electrodes 6 a and 6 b. Even if this insulating film ifleft as a sidewall, no problem is caused.

Third Embodiment

FIGS. 9A and 9B are a plane view of an MISFET in a semiconductor deviceaccording to a third embodiment of the present invention and across-sectional view illustrating a cross-sectional structure takenalong the line IX-IX (a cross section in the gate width direction),respectively. In this embodiment, a structure for generating a stress inthe gate width direction of a MISFET, i.e., in the direction which isparallel to the principal surface of a semiconductor substrate andvertical to the direction of movement of carriers will be described.

As shown in FIGS. 9A and 9B, in an active region 17 of a semiconductorsubstrate 1, i.e., an Si (100) substrate, provided is a MISFET includinga gate insulating film 5, a gate electrode 15 and source/drain regions 3and 4. The MISFET may be either an nMISFET or a pMISFET. Moreover, ateach of both ends of the gate electrode 15, first-type internal stressfilms 16 which spreads over part of side, end and upper surfaces of thegate electrode 15, the semiconductor substrate 1 and the isolationregion 2 and is made of a silicon nitride film are provided.

In this embodiment, a compressive stress is generated by each of thefirst-type internal stress films 16 at each end of each of thesource/drain regions 3 and 4 located directly under the first-typeinternal stress films 16. In regions of the source/drain regions 3 and 4on which the first-type internal stress film 16 is not located, atensile stress is generated in the gate width direction. Furthermore,with the gate electrode 15 having both ends thereof compressed by thefirst-type internal stress films 16, a tensile stress is generated inthe gate width direction in part of the gate electrode 15 on which thefirst-type internal stress films 16 are not located. As a result, in thechannel region 17 x, a stress due to the source/drain regions 3 and 4 towhich a tensile stress is applied and a stress due to the channel region17 x stretched by the gate electrode 15 to which a tensile stress isapplied are generated, so that a large tensile stress is generated inthe gate width direction. The direction of the tensile stress is theparallel direction to the principal surface of the semiconductorsubstrate 1 and also the vertical direction to the direction in whichcarriers move. Assume that an Si (100) substrate is used as thesemiconductor substrate 1 and the gate length direction is set to be the[011] direction. Whether carriers running through the channel region 17x are electrons or holes (i.e., in each of an nMISFET and a pMISFET),the mobility of carriers is improved due to the piezo resistivityeffect.

Note that in the structure shown in FIGS. 9A and 9B, the two first-typeinternal stress films 16 cover parts of the side and upper surfaces ofthe gate electrode 15 located at respective end portions of the gateelectrode 15. However, even if the two first-type internal stress films16 cover only the side surface of the gate electrode 15, a tensilestress in the gate width direction in the channel region 17 x can begenerated.

Furthermore, even in the case where the first-type internal stress film16 is provided only at one end of the gate electrode 15, a tensilestress at a certain level can be generated.

Moreover, in addition to this structure, another first-type internalstress film may be provided under each of end portions of the gateelectrode 15.

As has been described above, according to the present invention, astress is generated in a semiconductor device by an internal stressfilm, so that a stress can be generated in an arbitrary location on thesemiconductor substrate using semiconductor process steps which showaffinity to present process steps for fabricating a semiconductordevice, thus resulting in increase in the mobility of carriers.

1-14. (canceled)
 15. A semiconductor device, comprising a MISFET,wherein the MISFET includes: an active region made of a semiconductorsubstrate; a gate insulating film formed on the active region; a gateelectrode formed on the gate insulating film; source/drain regionsformed in regions of the active region located on both sides of the gateelectrode; and a silicon nitride film formed over from side surfaces ofthe gate electrode to upper surfaces of the source/drain regions,wherein the silicon nitride film is not formed on an upper surface ofthe gate electrode.
 16. The semiconductor device of claim 15, whereinthe silicon nitride film is for generating a stress in a substantiallyparallel direction to the gate length direction in a channel regionlocated in the active region under the gate electrode.
 17. Thesemiconductor device of claim 16, wherein the substantially paralleldirection of the stress includes a direction tilted by an angle of lessthan 10 degree from a direction in which carriers move.
 18. Thesemiconductor device of claim 15, wherein the silicon nitride film isdirectly in contact with the source/drain regions,
 19. The semiconductordevice of claim 15, wherein the silicon nitride film is formed above thesource/drain regions with a thin film interposed therebetween.
 20. Thesemiconductor device of claim 15, wherein the source/drain regionsinclude a lightly doped impurity region, a heavily doped impurity regionand a silicide layer.
 21. The semiconductor device of claim 15, furthercomprising: a sidewall formed on the side surface of the gate electrode.22. The semiconductor device of claim 15, wherein a principal surface ofthe semiconductor substrate is substantially a {100} plane, and the gatelength direction of the gate electrode is substantially a <011>direction.
 23. The semiconductor device of claim 15, further comprising:an interlevel insulating film formed on the silicon nitride film; and acontact plug provided so as to pass through the interlevel insulatingfilm and the silicon nitride film and to be connected to thesource/drain regions.
 24. The semiconductor device of claim 15, whereinthe active region is divided by an isolation region formed in thesemiconductor substrate.
 25. The semiconductor device of claim 15,wherein the gate insulating film is a silicon oxide film.
 26. Thesemiconductor device of claim 15, wherein the gate insulating film is asilicon oxynitride film.
 27. The semiconductor device of claim 15,wherein the gate electrode has a polysilicon film.
 28. The semiconductordevice of claim 15, wherein the gate electrode has a metal film.
 29. Thesemiconductor device of claim 15, wherein the silicon nitride film isprovided so as to cover at least part of at least one of thesource/drain regions.
 30. The semiconductor device of claim 15, whereinthe silicon nitride film covers at least respective parts of thesource/drain regions.
 31. The semiconductor device of claim 15, whereinthe silicon nitride film covers at least respective parts of both sidesurfaces of the gate electrode.
 32. The semiconductor device of claim15, wherein the MISFET is an nMISFET and the source/drain regions isn-type source/drain regions.
 33. The semiconductor device of claim 32,wherein the silicon nitride film is for generating a tensile stress in asubstantially parallel direction to the gate length direction in achannel region located in the active region under the gate electrode.34. The semiconductor device of claim 32, wherein the n-typesource/drain regions includes an n-type lightly doped impurity region,an n-type heavily doped impurity region and a silicide layer.